Get column name where value is something in pandas dataframe

Here is one, perhaps inelegant, way to do it:

df_result = pd.DataFrame(ts, columns=['value'])

Set up a function which grabs the column name which contains the value (from ts):

def get_col_name(row):    
    b = (df.ix[row.name] == row['value'])
    return b.index[b.argmax()]

for each row, test which elements equal the value, and extract column name of a True.

And apply it (row-wise):

In [3]: df_result.apply(get_col_name, axis=1)
Out[3]: 
1979-01-01 00:00:00    col5
1979-01-01 06:00:00    col3
1979-01-01 12:00:00    col1
1979-01-01 18:00:00    col1

i.e. use df_result['Column'] = df_result.apply(get_col_name, axis=1).

.

Note: there is quite a lot going on in get_col_name so perhaps it warrants some further explanation:

In [4]: row = df_result.irow(0) # an example row to pass to get_col_name

In [5]: row
Out[5]: 
value    1181.220328
Name: 1979-01-01 00:00:00

In [6]: row.name # use to get rows of df
Out[6]: <Timestamp: 1979-01-01 00:00:00>

In [7]: df.ix[row.name]
Out[7]: 
col5    1181.220328
col4     912.154923
col3     648.848635
col2     390.986156
col1     138.185861
Name: 1979-01-01 00:00:00

In [8]: b = (df.ix[row.name] == row['value'])
        #checks whether each elements equal row['value'] = 1181.220328  

In [9]: b
Out[9]: 
col5     True
col4    False
col3    False
col2    False
col1    False
Name: 1979-01-01 00:00:00

In [10]: b.argmax() # index of a True value
Out[10]: 0

In [11]: b.index[b.argmax()] # the index value (column name)
Out[11]: 'col5'

It might be there is more efficient way to do this...

Just wanted to add that for a situation where multiple columns may have the value and you want all the column names in a list, you can do the following (e.g. get all column names with a value = 'x'):

df.apply(lambda row: row[row == 'x'].index, axis=1)

The idea is that you turn each row into a series (by adding axis=1) where the column names are now turned into the index of the series. You then filter your series with a condition (e.g. row == 'x'), then take the index values (aka column names!).